The present invention relates generally to methods for controlling spin reorientation of spinning spacecraft. More specifically, the present invention relates to affecting a spin transition of a spacecraft from a minor axis spin to a particular major axis spin orientation.
A single-body spacecraft having energy dissipation and which spins about a minor axis is directionally unstable. Active control, such as rocket motors or dampers on despun platforms, stabilize such a spacecraft. Removal of these stabilizing mechanisms permit the spacecraft to reorient its principal spin axis and begin to rotate about its major axis. The reorientation results from energy dissipation, including fuel slosh and vibration. Spin transition is the term coined for this reorientation maneuver from a spin bias about a minor spin axis to a spin bias about a major spin axis.
FIG. 1 is a graphical representation of a spacecraft 10 with a major axis X, an intermediate axis Y, and a minor axis Z. The major axis X coincides with a principal moment of inertia axis having the largest inertia, I.sub.1. The minor axis Z is orthogonal to the major axis X and coincides with a principal moment of inertia axis having the smallest inertia, I.sub.3. The intermediate axis Y is orthogonal to both the major axis X and the minor axis Z and coincides with a principal moment of inertia having an intermediate inertia, I.sub.2. For virtually all real world spacecraft, I.sub.1 &gt;I.sub.2 &gt;I.sub.3.
Spin rates, either positive or negative, about the X axis, Y axis, or the Z axis are, respectively, .omega..sub.1, .omega..sub.2, and .omega..sub.3. Spacecraft often are purposefully and initially rotated about the minor axis Z for several reasons. First, launch vehicles have fairings which constrain or require that a long and narrow axis of the spacecraft, that is, the Z axis, be aligned with the longitudinal axis of the launch vehicle. The launch vehicle typically spins longitudinally prior to separation and results in a minor axis spin rate for the spacecraft 10 after separation. Second, when a rocket motor is used for orbit raising, the rocket motor and spacecraft combination is spun about the minor axis to increase stability during the firing and orbit raising. When the firing is completed, the combination undergoes spin transition.
Spin transition has some problems. One problem is that orientation of the spacecraft relative to an inertially-fixed angular momentum vector at the completion of the spin transition maneuver cannot be determined in advance with any degree of accuracy. In other words, the spacecraft 10 has either a positive or a negative spin about the major axis X. Physically, this corresponds to two final attitudes which are 180.degree. apart. Either face A or face B (of FIG. 1) will present itself in the desired direction. Many spacecraft 10 have sensitive on-board instruments which must be shielded from the sun or have directional communication equipment which must point towards the earth. In both of these situations, ensuring a final spin polarity about the major axis is necessary.
Prior art techniques of optimal reorientation for a spacecraft 10 for attitude acquisition using momentum wheels have been developed. In terms of fuel usage, the passive sign transition maneuver is optimal, and momentum wheels, with their attendant complexity, are not required. To make the maneuver truly useful, however, the final spin polarity must be controlled, requiring some fuel expenditure. Conventional methods attempt to control the spacecraft during spin transition to prevent occurrence of an improper orientation.